Grids for selective transmission of electromagnetic radiation may be used for example in medical imaging devices such as Computed Tomography scanners (CT), standard X-ray scanners like C-arm devices or mammography devices, Single Photon Emission Computed Tomography devices (SPECT) or Positron Emission Tomography scanners (PET). Other devices such as non-destructive X-ray testing devices may also use such grids. The grid may be positioned between a source of electromagnetic radiation such as an X-ray radiation source and a radiation-sensitive detection device. For example, in a CT scanner, the source of electromagnetic radiation may be an X-ray tube whereas in SPECT/PET a radioactive isotope injected into a patient may form a source of electromagnetic radiation. The radiation-sensitive detection device may be any arbitrary radiation detector such as a CCD-device, a scintillator based detector, a direct converter etc.
A grid may be used to selectively reduce the content of a certain kind of radiation that must not impinge onto the radiation-sensitive detection device. In a CT scanner, a grid may be used to reduce the amount of scattered radiation that is generated in an illuminated object and which may deteriorate the medical image quality. As today's scanners often apply cone-beam geometry, hence illuminate a large volume of an object, the amount of scattered radiation is often superior to the amount of the medical information carrying non-scattered primary radiation. For example, scattered radiation can easily amount to up to 90% or more of the overall radiation intensity, depending on the object.
Therefore, there may be a demand for grids that efficiently reduce scattered radiation. Grids fulfilling such demand may have radiation absorbing structures in two dimensions that form a selective transmission structure and that are called two-dimensional anti-scatter-grids (2D ASG). As such two-dimensional anti-scatter-grids may need to have transmission channels that are focused to a focal spot of the radiation source that emits the primary radiation which shall be allowed to be transmitted through the grids, the grid may have to exhibit a sophisticated geometrical structure and it may be complicated, time-consuming and costly to manufacture such grid.
WO 2008/007309 A1, filed by the same applicants as the present application, describes a grid for selective transmission of electromagnetic radiation with structural elements built by selective laser sintering. Therein, a method for manufacturing a grid comprises the step of growing at least a structural element by means of selective laser sintering from a powder material, particularly a powder of an essentially radiation-opaque material. Selective laser sintering allows for a large design freedom. Having a structural element that is built by selective laser sintering, the grid may be a highly complex three-dimensional structure that is not easily achievable by conventional moulding or milling techniques. Therein, the technology of selective laser sintering, sometimes also known as direct metal laser sintering, is not any longer a prototype technology but becomes a production technology for the manufacturing of three-dimensional devices with demanding geometries.
For CT-applications the typical dimension of a footprint of a two-dimensional grid is in the range of 2 cm×2 cm for a modular detector system with a height of for example about 27 mm.
In contrast hereto, the dimension of a grid typically used for mammography applications may be in the range of e.g. 18 cm×24 cm which is a much bigger footprint but the height is only about 2 mm. At the same time the typically required wall thickness may be even smaller compared to the wall thickness of CT grids which are in the range of about 100 μm.
A problem for the production of such large footprint grids may be the separation of the grid from a metal carrier that is normally used for smaller devices.
A device with a 2 cm×2 cm footprint can be separated from the carrier by dicing, wire erosion or even by producing a kind of perforation, which may be achieved by not continuously sintering the sinter material by stopping the laser power appropriately to establish a predetermined breaking point.
However, for grids usable for example in mammography applications having a large footprint and a low thickness it might be complicated to, after preparation using selective laser sintering, separate the grid from the underlying metal carrier. For example, with respect to the thickness of such grid being about 2 mm, material losses induced by the separation process using dicing, which may be in the range of about 1 mm thickness, are a significant loss in both, sinter material and work effort. Furthermore, even if it would be possible to efficiently separate the grid from the necessary carrier or building platform, the handling of such thin and not very stable grid layer might be an issue.